1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (LCD) device and, more particularly, to a driving technique for driving the active-matrix LCD panel in the LCD device.
2. Description of the Related Art
An in-plane switching (IPS) mode active-matrix LCD (AM-LCD) panel uses a lateral electric field for driving the LC layer, the lateral electric field being parallel to the interface between the LC layer and the substrate surface. In the IPS mode AM-LCD panel, the LC layer should have a lower specific resistivity in order to prevent the residual image of the previous image from being displayed on the screen for a long time (See JP-A-7-159786, for example). On the other hand, in a general AM-LCD panel, irrespective of either the IPS mode or a twisted nematic (TN) mode of the LCD panel, a mode switching function is generally used for switching between the display modes in resolution or for switching between two set of input sections.
For example, in the mode switching function between the display modes, either a higher resolution mode (SXGA) for displaying 1280xc3x971024 pixels or a lower resolution mode (VGA) for displaying 640xc3x97480 pixels is selected on the screen by the user.
In the AM-LCD panel, the image signal is not supplied to the LCD panel for several seconds during the mode switching operation between the display modes or between the input sections. If the screen continues for displaying image during this switching period, large noise appears on the screen to degrade the image quality. Thus, it is generally employed in the AM-LCD to temporarily stop displaying the image on the screen during the switching operation.
Immediately after the LCD panel is restarted for image display following to the temporary stop of the image display, there is a phenomenon wherein a flicker appears on the screen for about 2 to 20 seconds. The cause of the flicker is considered as follows.
In a LCD device, an alternately driving technique is generally used for preventing degradation of the LC layer such as burning. In other words, the potential of the pixel electrode alternates in the polarity thereof with respect to the potential of the common electrode (or common electrode potential). For temporarily stopping the image display on the screen during the switching operation, it may be considered to equalize the pixel electrode potential with the common electrode potential at the ground potential, for example. However, this causes a difference in DC potential between both the electrodes.
The difference in the DC potential (DC potential difference) is caused by the fact that the common electrode immediately assumes the ground potential whereas the pixel electrode does not assume the ground potential during the switching operation due to a large discharge period of the pixel electrode. This is because the pixel electrode is grounded via the data line and the TFT in the pixel, which delay the discharge (or drainage) of the electric charge from the pixel electrode.
The DC potential difference reverses from pixel to pixel in the case of a dot reversible driving technique of the LCD panel, wherein the image signal is reversed in the polarity thereof pixel by pixel in both the column and row directions and also frame by frame in each pixel. The DC potential difference causes attachment of electric charge having an opposite polarity with respect to the polarity of the potential difference and an amount corresponding to the potential difference onto each of the pixel electrodes or the common electrodes. The attached electric charge generally remains on the electrode as residual electric charge after the LCD panel is restarted for image display from the temporary stop in the mode switching operation. The residual electric charge is superposed with the current image signal and lightens or darkens the screen every frame, thereby causing a flicker until the residual electric charge eventually disappears. The flicker has become more noticeably with the reduction of the specific resistivity of the LC layer.
The mechanism of the flicker problem will be described with reference to drawings. FIG. 1 shows an equivalent circuit diagram of each pixel in the pixel array of the LCD panel. The pixel includes a parallel branch including LC layer capacitor CL and LC layer resistor RL and connected through a capacitor C1 to the pixel electrode 210 and through a capacitor C2 to the common electrode 212. The capacitors C1 and C2 are formed by the LC layer and the pixel electrode 210 and the common electrode 212, respectively, sandwiching therebetween a passivation layer 213 as shown in FIG. 2A (and FIG. 2B) The pixel electrode 210 is connected to the source of TFT 206, the drain of which is connected to a corresponding data line. The gate of TFT 206 is connected to a corresponding gate line 202, whereas the common electrode 212 is connected to the common electrode line 204.
When a DC voltage is applied between the pixel electrode 210 and the common electrode 212 during the switching operation for resolution mode or selection of input signals, with the pixel electrode 210 having a positive polarity with respect to the common electrode 212, positive electric charge in the LC layer moves toward the common electrode 212 as schematically shown by an arrow in FIG. 2A. The positive electric charge, after reaching the vicinity of the common electrode 212, forms a residual electric charge and applies an electric field in the direction from the common electrode 212 to the pixel electrode 210 as shown by an arrow in FIG. 2B.
FIG. 3 shows a timing chart of potentials of electrodes and nodes in the LCD panel of FIG. 1 for a switching operation, wherein the input of the data line is switched from an input signal A to an input signal B. In FIG. 3, gate line potential, data lien potential, pixel electrode potential and common electrode potential are represented by VG, VD, VPI (VPIxe2x80x2) and Vcom. After the input signal A having a positive polarity is input through the data line, the potential VG of the gate line 202 rises from a Vgoff level (xe2x88x9210 volts, for example) to a Vgon level (19 volts, for example) at time instant t1, whereby the TFT 206 is turned on to deliver the input signal A to the pixel electrode, which holds the voltage level until the power supply is switched off at t6. In FIG. 3, only a bias voltage applied to the data line is depicted as the potential VD of the data line, with the signal voltage superposed thereon being omitted
The potential VG of the gate line 202 rises at t2 from Vgon level to Vgoff level, which is maintained on the gate line 202 until t6. The common electrode potential Vcom assumes a constant level (4.5 volts, for example) until t6. The input signal A is switched off by a data driver at t3 from the data line 200, although the voltage level VD is maintained on the data line 200 until t4, at which a an absence signal is delivered indicating that the input signal is not supplied from the data driver.
The time period T1 between time t3 at which the input signal A is switched off and time t4 at which the absence signal is delivered corresponds to the time needed for the video signal processor receiving the input video signal to judge the absence of the input signal, and may be 40 milliseconds, for example. The time period T2 between time t4 and time t6 at which the power supply for the LCD panel is switched off corresponds to the time needed for switching off the power supply after the judgement of the absence of the input signal, and may be 5 milliseconds, for example. The time period T3 between time t6 and time t7 at which the input signal B is switched on corresponds to a waiting time for waiting a new input signal, and may be 300 milliseconds, for example.
The power source for the data driver is switched off at time t4 at which the absence signal is delivered indicating the absence of the input signal, whereby the potential of the data line falls and assumes the ground potential at time t5.
At time t6, the power source for the LCD device is switched off, and the gate line 202 and the common electrode line 204 are directly aplied with the ground potential, whereby the gate line potential VG and the common electrode potential Vcom immediately assume the ground level whereas the pixel electrode potential VPI gradually falls to the ground level due to the discharge via the TFT. This causes a positive DC voltage difference xcex94V between the pixel electrode 210 and the common electrode 212.
On the other hand, if the input signal A has a negative polarity in the pixel, a negative DC voltage difference xe2x88x92xcex94V is generated between the potential VPIxe2x80x2 of the pixel electrode 210 and the potential Vcom of the common electrode 212, as shown in FIG. 3. These DC voltage differences xcex94V and xe2x88x92xcex94V cause flicker on the screen due to the attachment of the electric charge having an opposite polarity to the vicinity of the pixel electrode 210.
It is assumed that the LCD device uses a dot reversible driving technique, and that the polarities of the pixel electrodes for a group of pixels after the input signal A is turned off and before the input signal B is switched on are such that shown in FIG. 4A, that is, the group (3xc3x973) of pixel electrodes stores electric charges in the staggered pattern shown in FIG. 4A. In the assumed case, the pattern of the residual charges for the pixel is the inverse of the staggered pattern of FIG. 4A, as shown in FIG. 4B, whereby the residual charges cancel or reduces the potentials held on the pixel electrodes.
After the input signal B is switched on in the subsequent frame, the polarities of the pixels are reversed from the pattern of the input signal in the pervious frame shown in FIG. 4A, whereby the potentials of the input signals supplied to the pixel electrodes are intensified by the residual charges, as shown in FIG. 4C. In the further subsequent frame, the input signal pattern changes to the pattern shown in FIG. 4B due to the reversible driving technique, whereby the residual charge cancels or reduces the potentials of the input signals on the pixel electrodes. In other words, the residual charges reduce the signal charges in the odd-numbered frames, whereas the residual charges intensify the signal charges in the even-numbered frames. This causes a flicker having a frequency corresponding to the frame frequency, is more likely to generate in the case of IPS mode LCD panel. The flicker is observed more noticeably in the case of a LC layer having a lower specific resistivity, especially in the case of specific resistivity of 1xc3x971013 xcexa9-cm or lower due to a higher mobility of the charge in the LC layer and a higher electric field generated by the residual charges
A similar flicker problem arises in the case of a frame reversible driving scheme, a scanning-line reversible driving scheme, and a data-line reversible driving scheme other than the dot reversible driving scheme. The term xe2x80x9creversible driving schemexe2x80x9d as used in this text without addition of any modification means either the dot reversible driving scheme, the scanning-line reversible driving scheme or the data-line reversible driving scheme.
In view of the above, it is an object of the present invention to provide an AM-LCD device which is capable of preventing a flicker during the switching operation for the resolution modes or the input signals.
The present invention provides an active-matrix LCD (AM-LCD) device including a pixel array including a plurality of pixels arranged in a matrix, each of the pixels having a pixel transistor, a pixel electrode connected to a source of the pixel transistor and a common electrode, a plurality of data lines each connected to a drain of each pixel transistor arranged in a column of the pixel array, a plurality of gate lines each connected to a gate of each pixel transistor arranged in a row of the pixel array, a data drive block for driving the data lines, a gate driver block for driving the gate lines, a switching section for switching power source for the pixel array, a control section for controlling the data driver block, the gate driver block and the switching section to drive the pixel array in a reversible driving scheme, wherein the control section controls a mode switching so that a potential difference between the pixel electrode and the common electrode in each pixel has a uniform polarity among the pixels during a switching period after image data for the data line is switched off and before the power source for the pixel array is switched on for the mode switching.
In accordance with the LCD device of the present invention, the same polarity of the potential difference between the pixel electrode and the common electrode among the pixels affords reduction of the flicker caused by the residual charge.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.